This invention relates to an NMR imaging method using nuclear magnetic resonance (hereinbelow abbreviated to NMR) and an apparatus for realizing same, and in particular to an NMR imaging method and an apparatus for realizing same permitting to remove artifact originating from an object body to be examined outside of a uniform magnetic field.
In an NMR imaging method and an apparatus therefor it is necessary to separate and recognize signals coming from an object body to be examined, making them correspond to the position of each part in the object body to be examined. As a method therefor there is known a method, by which different magnetic fields are applied to different parts of the object body to be examined and positional information is obtained by making the resonance frequency or the phase shift quantity at different parts of the object body to be examined different in this way. FIG. 2 shows schemes for explaining the principle of the method described above and represents variations in high frequency magnetic field, gradient magnetic field and detection signal with respect to time. That is, a high frequency magnetic field and a gradient magnetic field, which are applied to the object body to be examined and an NMR signal originating from the object body to be examined are illustrated there. This method is named Fourier zeugmatography method by Kumar et al. and its fundamental principle is described in Journal of Magnetic Resonance (18, 69-83 (1975)). The spin warp method, which is a modification thereof, has been also proposed (Phys. Med. Biol. 25,751 (1980)) and starting therefrom, various changes and improvements have been made thereon.
For the imaging of a spatially large portion such as a human body it is necessary to generate a static magnetic field as strong as 0.05-2 Tesla with a uniformity less than several tens of ppm in a spherical space, whose diameter is as large as 30-50 cm. For this reason, in many NMR imaging apparatuses the cost of the static magnetic field generating section covers the greater part of the total cost. There are known various methods for generating a uniform and large magnetic field, but basically more larger the uniform space is, the higher the cost is. Therefore it is conceivable to restrict the uniform space to a practical extent in order to reduce the cost. However, this method has a serious problem that intense artifact from the object body outside of the uniform magnetic field is produced. FIGS. 3-5B are schemes for explaining this problem in connection with the imaging method. For the convenience' sake a two-dimensional spin warp method is taken as an example, but it will be easily inferred from the following explanation that similar problems are produced by other methods. FIG. 3 shows the distribution of a static magnetic field. In order to facilitate the understanding, it is supposed that the magnetic field is uniform within a sphere having a radius r and decreases or increases rapidly outside thereof. The object body to be examined is located in this static magnetic field and at first a slice plane is selected by applying a high frequency magnetic field having a restricted band and a gradient magnetic field G.sub.x at the same time. FIGS. 4A and 4B show circumstances at this time. FIG. 4A indicates that because of the fall of the static magnetic field selective excitation takes place not only at the slice plane x=x.sub.s .+-..DELTA.x, which is to be aimed, but also at the neighborhood of x=x.sub.a. FIG. 4B indicates the part (hatched part in the figure), where the selective excitation really takes place, in the X-Y plane at Z=0. Here the linear part, which is parallel to the y axis in FIG. 4B, indicates the position of the slice plane, which is the object to be examined, and it is necessary to pay attention to the fact that the selective excitation takes place also at the curved part, which is not intended, at the same time. These circumstances are produced not only on the part at Z=0 but also on a three-dimensional curved surface including the Z direction, depending on variations in the static magnetic field. Subsequently a gradient magnetic field G.sub.z is applied and the phase encode is effected in the Z direction. Finally signals are observed while applying a gradient magnetic field G.sub.y thereto. At this time the frequency of the signal varies, depending on the intensity of the magnetic field, which is proportional to the position on the Y axis, and positional information in the Y direction is expressed in the form of differences between frequencies. However, because of the fall of the static magnetic field just as for the selection of the slice plane, as indicated in FIG. 5A, there is a part at another position on the Y axis, where a signal having the same frequency is produced. FIG. 5B shows circumstances on the X-Y plane at Z=0, and in connection with the selective excitation plane indicated in FIG. 4B, the curved surface portion right above in the hatched part in FIG. 5B emits non-intended signals. There circumstances take place on a three-dimensional curved surface, depending on variations in the static magnetic field, just as for the selection of the slice plane. If the object body exists also in this part, the image of the curved surface part is superposed on the properly so called reconstructed image of the slice plane and gives rise to strong artifact. Although it is supposed that the magnetic flux density decreases outside of the uniform magnetic field, in order to facilitate the understanding, and further that the gradient magnetic fields are ideally linear systems, it is easily inferred that also in the case where these two magnetic fields have other non-linear components, only the form of the curved surface varies and similar problems take place.
In a prior art device for imaging the human body, in order to avoid this problem, a uniform space, whose diameter is longer than 40 cm, is necessary, and even if such a uniform space is obtained, since the stature of the human body is longer than this size, artifact due to this fact may occur in parts above and/ or below, or on the left and/or right sides of the image. Furthermore the cost reduction owing to the method, by which the uniform space is reduced, was practically impossible by the prior art method.